A dipstick is one of several devices dipped into a liquid to perform a chemical test or to provide a measure of quantity of the liquid.
This is a common way to measure the amount of oil in the sump of an internal combustion engine. The dipstick is a metal strip or thin flexible coil inserted into a tube that passes through the engine block into the sump. The dipstick will usually have minimum and maximum level marks. A car's transmission fluid reservoir sometimes has a similar, though usually longer, device.
To check the oil the vehicle should be parked on a level surface and allowed to stand with the engine off so the oil can drain into the sump. Withdraw the dipstick and wipe it with a clean rag. Do not read the stick yet, as splashed oil would give an erroneously high reading. Insert the dipstick fully into its tube, withdraw it again and read it.
An oil dipstick also provides a handy way to spot dirt and contaminants.